General comments

This paper presents data from about a 9 year period showing a general decrease in black carbon aerosol in Beijing and the implications of that decrease for single scattering albedo and radiative forcing.  Overall, this work provides a useful study for publication, but a number of items should be addressed in the final draft.

First, carefully proofread the paper for grammar, proper word choice, etc.  I list a few things below, but I do not list all the errors in the current draft.

Second, the MRS method is newly developed and not cited much; thus, I think it would be appropriate to provide a bit more detail in your paper on how this method separates primary and secondary BrC.  If necessary, provide some graphs in the Supplemental.

Likewise, I do not understand the Monte Carlo simulations of uncertainties on radiative forcing.  Please explain this process better.

Regarding data interpretation, please comment on the fact that in Figure 1, the major downshift in eBC concentration corresponds to the switch from one aethalometer to the other.  Did the old model have a higher uncertainty or more noisy signal?  Please detail the extra correction you refer to in line 93, as this correction plays a role in how these data look on Fig 1.

It is quite interesting that the eBC and CO both drop by nearly exactly the same percentage from 2012 to 2020, and yet the emission ratio of delta eBC / delta CO also changes, thus indicating a change in primary emission source.  My question is in regards to how you calculate the emission ratio - why do you use a mean or initial eBC value of zero while you don't for CO?  Provide justification in the text.  Also, if there is any uncertainty to performing the calculation this way, what effect would that have on your final emission ratio trends?

Talk more about the possible emission sources and how they match to your emission ratio calculation.

You mention biomass burning as a significant source of BC, and we see in the data that BC emissions are decreasing over time.  Can you elaborate on what types of biomass burning:  wildfires, prescribed fires, residential heating, etc. are prevalent in the study area or may be transported over the site?

Why does the change in primary emission come in 2018, when the clean action plan was for 2013-2017?  Did nothing happen until the end of the 5 year plan?

There are a number of places that say ``similar to previous studies'', or something like that.  Can you also highlight what is new and different about your measurements that make your study unique?

Discuss Fig 10 some more.  How significant are these numbers in terms of the total radiative balance?  What implications for long-term changes to climate do these numbers show?

Specific comments

Be precise about how you refer to ``9 years of data''.  There are actually large gaps in your dataset, as presented in Fig S1.

Be consistent with how you refer to black carbon aerosol - you go back and forth between ``BC'' and ``eBC''.  I believe you are actually measuring eBC as you defined in the methods section.  Make sure to fix the abstract also.

A map of the study area showing the sampling locations would be nice and would provide context for Fig 5.

Are b_{abs} and b_{abs,BC} the same thing?  I'm looking at equations 2 and 5. 

Lines 145, 148, 150 - why aren't these equations separated and numbered like others in the paper?

Lines 157 and 179 are essentially the same sentence.

Fig 1 - the triangles are hard to see.  Also, ``BC'' or ``eBC''?

Fig 2 - need error bars showing uncertainty or scatter in the data.  Otherwise, we don't know how significant the monthly variations are.

Line 175 - actually 2014 had an increase too

Should ``clean air action plan'' be capitalized as a specific document or law?

Line 215 - Why is 2012 an exception?

Line 218 - Why is regional transport only important some of the time?

Line 315 - How do you know 2020 had a stronger atmospheric oxidation capacity?

Figure 5 - Why not use the same color scale on each plot?

Is it necessary to show the top rows of Figures 6 and 7?  These are just Figures 3 and 4 multiplied by a constant and therefore don't show anything new.

Fig 8 - the colors are hard to see

Technical comments

There are quite a few technical corrections to be made for grammar, word choice, etc.  Only a few will be listed here; there are too many to write all of them out.  One common mistake is missing articles such as ``the''.  Another common confusion is the use of ``this'' and ``it'' as subjects in a sentence when these terms are no always clear as to what they are referring to.

Line 244 - do you mean Fig 6?

Line 269 - ``similar'' to what?

Line 307 - ``similar'' to what?

Line 351 - ``POA'' is never defined

Check the bibliography carefully; there are formatting errors throughout.

The manuscript by Sun et al. investigated black carbon and aerosol optical properties from 2012 to 2020 in Beijing, which is an important work to understand long-term changes of black carbon, aerosol optical properties and radiative effects after clean air action. Here, the authors applied a relatively mature experimental design to conduct a long-term observation. The results showed that large reductions in eBC and light extinction coefficient of fine particles were about 67 % and 47 %, respectively. And the most significant reductions of eBC were occurred in the fall and night time, mainly due to the changed primary emissions. Comparatively, SSA and MEE considerable increases highlight an increasing importance of scattering aerosols in radiative forcing, and a future challenge in visibility improvement due to enhanced MEE. Besides, the authors also quantified the primary and secondary BrC, demonstrating an enhanced role of secondary formation in BrC in recent years. In the last, the influences on direct radiative forcing from changes in BC and BrC are estimated. The whole result analysis in this study is systematical, thoughtful and in a well-organized manner. According to the state of the art, and the conclusions are supported by the data, so I consider that it fulfils the necessary requirements to be published. I recommend it for publication on Atmospheric Chemistry and Physics after the authors consider several minor revisions to the manuscript.

Comments:

Please clarify eBC in abstract.

line 30&34, “during 2018-2020” format should be consistent, please check the full text.

line 63, “accounting for 25 ~ 35 % 2010” should be “accounting for 25 ~ 35 % in 2010”.

line 84, ignore the punctuation. Please check the full text.

line 107, Please check the Spaces after commas in the right subscript of the full text (e.g., babs, BC).

line 123, “the correlation (R2)” should be “the correlation coefficients (R2)”.

Line 160-170, it would be helpful to compare with a study about the seasonal variation of ΔBC/ΔCO in Beijing influenced by vertical transport of BC and wet scavenging. (Black carbon emission and wet scavenging from surface to the top of boundary layer over Beijing region, JGR-atmosphere, 125(17), 10.1029/2020JD033096, 2020.)

line 163, Change “-” to “–”

line 186 ~ 187, lack a preposition in this sentence. Please rewrite.

line 193, “the diurnal variation of eBC (Fig. 4) showed morning peaks” should be “the diurnal variations of eBC (Fig. 4) showed morning peaks”.

line 208, diurnal variation of winter in 2020 was not showed in Fig. 4. Please rewrite this sentence.

line 209, “One reason was likely due to the fact that” could be “One reason was likely because”

line 214, “the eBC presented similar distribution with high concentration in the middle and also the… “should be “the eBC presented similar distribution with high concentration in the middle and the…”.

line 241, “is close to” should be “was close to”.

line 244, “Fig. 7” should be “Fig. 6”.

Please check the spaces of the full text (like line 107, e.g., babs, BC).

Line 310-315, a study which observed enhanced BrC contribution on absorption when heavy pollution could be referenced. (In situ vertical characteristics of optical properties and heating rates of aerosol over Beijing, Atmospheric Chemistry and Physics, 20(4), 2603–2622, 10.5194/acp-20-2603-2020, 2020.)